The present invention can be applied to radio systems using directional antenna beams. The directional antenna beams are usually narrow beams pointing in a determined direction. The beams can be either fixedly directed, whereby the coverage area of a base station is covered by several radially directed beams, or, alternatively, the beams can be directed towards a particular terminal or particular terminals, and, in that case, if the terminal moves, the beam follows the location of the terminal. The method in which the users are distinguished from each other on the basis of their location is called an SDMA (Space Division Multiple Access) method. For this purpose, adaptive antenna arrays, i.e. phased antennas, and processing of the received signal enabling the mobile stations to be monitored are typically used in the base station. The SDMA enables the signal-to-interference ratio or the signal-to-noise ratio to be improved, for example in the base stations of mobile telephone systems.
A problem in the use of SDMA methods is how to determine subscriber terminal directions for the transmission of the base station. The narrower the beams desired to be used, the more accurate must be the direction determination. Narrowing the beams is, in turn, profitable, since it reduces disturbances.
It is previously known to determine the direction on the basis of a signal transmitted by the subscriber station and received by the base station, assuming that the radio channel is sufficiently similar in both transmission directions. Accurate determination of the direction in such a manner takes rather a long time, since interference and fading on the transmission path cause inaccuracy whose elimination requires the averaging of results. On the other hand, direction determination should be performed as fast as possible. This is essential particularly in fast packet-switched radio systems, since connections must be established fast, reliably and without spending frequency resources.